14.2 General Features of the Discharge Devices and Processes
515
and then they are kept separated typically with a gap of 0.25-3.0 mm between them.
For the maintenance of the discharge process, a voltage of 18 to 30 V is necessary that
leads to the flow of a very large current ranging from 30 to 200 A (the voltage tends to
increase with the electrode distance and the current increases with the anode diameter
under otherwise identical conditions) [4, 5, 18, 23–31]. The temperature in the core
of the discharge is around 4000 K. In most cases, the discharge can be maintained
for a few minutes. The arc discharge devices are equipped with a cooling system
(water pipelines) and an electrode adjustment mechanism that precisely maintains
a constant interelectrode gap which is the prerequisite for the discharge stability.
Direct current discharges represent an overwhelming majority of the experiments.
Although experiments with alternating current have also been reported [4, 32], this
method could not lead to an improvement in either the yield or the product selectivity.
Pulsed RF discharge is also mentioned occasionally [30, 33] where the maintenance
of the discharge requires a voltage in the range of 3.5–5.5 kV.
The electrode materials are mostly graphite rods. The diameter of the anode is
usually 6–8 mm, while the cathode is often larger and can be even a plate. When
carbon is used as anode material, it is consumed due to the impact of the energetic
electrons coming from the plasma, and the deposit forms at both the cathode and in
various parts of the discharge chamber. A major difference as compared to the experimental setups making use of intercalation and upcoming graphene sheet detachment
(discussed in Chap. 10.5) is that in the arc discharge experiment the carbon products
with sp
2 hybridization form fully independently of the same bond structure in the
anode since the high-temperature plasma leads to a nearly complete atomization.
Therefore, the original graphene sheets are not conserved, and the final products do
not form as a result of the roll-up of the original graphene sheets of the anode. This
is particularly true for the experiments in which the carbon source is a gas, not the
anode.
The electrodes are arranged as coaxially positioned rods in most of the setups
reported (180
o axis inclination). This is not the only possible electrode arrangement
but configurations with an as low as 30
o electrode inclination are also possible. This
configuration was termed as arc jet method [34], referring to the fact that the plasma
extends rather far from the interelectrode space. The inclined electrode configuration
could be applied in both gas [35] phase and liquid-submerged discharges [36, 37].
In order to increase the stability of the discharge, a configuration with rotating one
of the electrodes was developed. In accord with the concept of this arrangement [38],
the rotation of the anode distributes the microdischarges uniformly along the anode
surface, hence overwriting the impact of the lateral inhomogeneity of the electrodes,
which leads to a stable plasma. Beside the improvement in the plasma stability, an
increase of the product yield was also reported. Since the rotation could be increased
up to 10
5 rpm, the primary location of the product deposition changed from the
cathode to the cylindrical collector around the plasma.
Several attempts were published on the improvement of the arc discharge conditions by applying an external magnetic field [39–41], the maximum being as high as
10 T [42]. The impact of the external magnetic field manifests itself by the increase
in the plasma density and the localization of the product deposition.
515
and then they are kept separated typically with a gap of 0.25-3.0 mm between them.
For the maintenance of the discharge process, a voltage of 18 to 30 V is necessary that
leads to the flow of a very large current ranging from 30 to 200 A (the voltage tends to
increase with the electrode distance and the current increases with the anode diameter
under otherwise identical conditions) [4, 5, 18, 23–31]. The temperature in the core
of the discharge is around 4000 K. In most cases, the discharge can be maintained
for a few minutes. The arc discharge devices are equipped with a cooling system
(water pipelines) and an electrode adjustment mechanism that precisely maintains
a constant interelectrode gap which is the prerequisite for the discharge stability.
Direct current discharges represent an overwhelming majority of the experiments.
Although experiments with alternating current have also been reported [4, 32], this
method could not lead to an improvement in either the yield or the product selectivity.
Pulsed RF discharge is also mentioned occasionally [30, 33] where the maintenance
of the discharge requires a voltage in the range of 3.5–5.5 kV.
The electrode materials are mostly graphite rods. The diameter of the anode is
usually 6–8 mm, while the cathode is often larger and can be even a plate. When
carbon is used as anode material, it is consumed due to the impact of the energetic
electrons coming from the plasma, and the deposit forms at both the cathode and in
various parts of the discharge chamber. A major difference as compared to the experimental setups making use of intercalation and upcoming graphene sheet detachment
(discussed in Chap. 10.5) is that in the arc discharge experiment the carbon products
with sp
2 hybridization form fully independently of the same bond structure in the
anode since the high-temperature plasma leads to a nearly complete atomization.
Therefore, the original graphene sheets are not conserved, and the final products do
not form as a result of the roll-up of the original graphene sheets of the anode. This
is particularly true for the experiments in which the carbon source is a gas, not the
anode.
The electrodes are arranged as coaxially positioned rods in most of the setups
reported (180
o axis inclination). This is not the only possible electrode arrangement
but configurations with an as low as 30
o electrode inclination are also possible. This
configuration was termed as arc jet method [34], referring to the fact that the plasma
extends rather far from the interelectrode space. The inclined electrode configuration
could be applied in both gas [35] phase and liquid-submerged discharges [36, 37].
In order to increase the stability of the discharge, a configuration with rotating one
of the electrodes was developed. In accord with the concept of this arrangement [38],
the rotation of the anode distributes the microdischarges uniformly along the anode
surface, hence overwriting the impact of the lateral inhomogeneity of the electrodes,
which leads to a stable plasma. Beside the improvement in the plasma stability, an
increase of the product yield was also reported. Since the rotation could be increased
up to 10
5 rpm, the primary location of the product deposition changed from the
cathode to the cylindrical collector around the plasma.
Several attempts were published on the improvement of the arc discharge conditions by applying an external magnetic field [39–41], the maximum being as high as
10 T [42]. The impact of the external magnetic field manifests itself by the increase
in the plasma density and the localization of the product deposition.